Field of the Invention
The present invention relates to systems, methods and apparatuses for Fischer-Tropsch gas to liquid hydrocarbon production. Specifically, the present invention relates to systems, methods and apparatuses for establishing catalyst activation and/or regeneration in a Fischer-Tropsch system.
Background of the Invention
The Fischer-Tropsch (or “Fischer Tropsch,” “F-T” or “FT”) process (or synthesis or conversion) involves a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen (known as reformed gas, synthesis gas, or “syngas”) into liquid hydrocarbons (called “liquid FT hydrocarbons” herein). The liquid FT hydrocarbons may include a wax (“FT wax”) that may be liquid when produced but becomes solid as it cools. The process was first developed by German chemists Franz Fischer and Hans Tropsch in the 1920's. The FT conversion is a catalytic and exothermic process. The FT process is utilized to produce petroleum substitutes, typically from carbon-containing energy sources such as coal, natural gas, biomass, or carbonaceous waste streams (such as municipal solid waste) that are suitable for use as synthetic fuels, waxes and/or lubrication oils. The carbon-containing energy source is first converted into a reformed gas, using a syngas preparation unit in a syngas conversion. Depending on the physical form of the carbon-containing energy source, syngas preparation may involve technologies such as steam methane reforming, gasification, carbon monoxide shift conversion, acid gas removal, gas cleaning and conditioning. These steps convert the carbon source to simple molecules, predominantly carbon monoxide and hydrogen, which are active ingredients of synthesis gas. Syngas also contains carbon dioxide, water vapor, methane, and nitrogen. Impurities deleterious to catalyst operation such as sulfur and nitrogen compounds are often present in trace amounts and are removed to very low concentrations, often as part of synthesis gas conditioning. Once the syngas is created and conditioned, the conditioned syngas is used as an input to an FT reactor having an FT catalyst to make the liquid FT hydrocarbons in a Fischer-Tropsch synthesis process. Depending on the type of FT reactor, the FT conversion of the syngas to liquid FT hydrocarbons takes place under appropriate operating conditions.
Depending on the physical form of the carbon-containing energy source, syngas preparation may involve technologies such as steam methane reforming, gasification, carbon monoxide shift conversion, acid gas removal gas cleaning and conditioning. These steps convert the carbon-containing energy source to simple molecules, predominantly carbon monoxide and hydrogen, which are the active ingredients of synthesis gas. The synthesis gas will also contain carbon dioxide, water vapor, methane, nitrogen. Impurities deleterious to catalyst operation such as sulfur and nitrogen compounds are often present in trace amounts and are removed to very low concentrations as part of synthesis gas conditioning.
Turning to the syngas conversion step, to create the syngas from natural gas, for example, methane in the natural gas reacts with steam and oxygen in a syngas preparation unit to create syngas. The syngas comprises principally carbon monoxide, hydrogen, carbon dioxide, water vapor and unconverted methane. When partial oxidation is used to produce the synthesis gas, typically, the syngas contains more carbon monoxide and less hydrogen than is optimal and consequently, steam is added to the react with some of the carbon monoxide in a water-gas shift reaction. The water gas shift reaction can be described as:CO+H2O⇄H2O+CO2  (1)
Thermodynamically, there is an equilibrium between the forward and the backward reactions. That equilibrium is determined by the concentration of the gases present.
Turning now to the FT conversion step, the Fischer-Tropsch (FT) reactions for the FT conversion of the syngas to the liquid FT hydrocarbons may be simplistically expressed as:(2n+1)H2+nCO→CnH2n+2+nH2O,  (2)where ‘n’ is a positive integer.
The FT reaction is performed in the presence of a catalyst, called a Fischer-Tropsch catalyst (“FT catalyst”). Unlike a reagent, a catalyst does not participate in the chemical reaction and is not consumed by the chemical reaction itself, but accelerates the chemical reaction. In addition, a catalyst may participate in multiple chemical transformations. Catalytic reactions have a lower rate-limiting free energy of activation than the corresponding un-catalyzed reaction, resulting in higher reaction rate at the same temperature. However, the mechanistic explanation of catalysis is complex. Catalysts may affect the reaction environment favorably, or bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause lysis of reagents to reactive forms, such as atomic hydrogen in catalytic hydrogenation.
In addition to liquid FT hydrocarbons, Fischer-Tropsch synthesis also commonly produces gases (“Fischer-Tropsch tail gases” or “FT tail gases”) and water ((“Fischer-Tropsch water” or “FT water”). The FT tail gases typically contain CO (carbon monoxide), CO2 (carbon dioxide), H2 (hydrogen), light hydrocarbon molecules, both saturated and unsaturated, typically ranging from C1 to C4, and a small amount of light oxygenated hydrocarbon molecules such as methanol. Typically, the FT tail gases are mixed in an FT facility's fuel gas system for use as fuel.
The FT water may also contain contaminants, such as dissolved hydrocarbons, oxygenates (alcohols, ketones, aldehydes and carboxylic acids) and other organic FT products. Typically, the FT water is treated in various ways to remove the contaminants and is properly disposed of.
A variety of FT catalysts is utilized to catalyze the Fischer-Tropsch synthesis, with cobalt-based catalysts and iron-based catalysts being the most prevalent. In addition, most FT catalysts are either supported or precipitated. If an FT catalyst is supported, a metal-based catalyst is deposited upon the interior of a metal structure resembling a tunnel, having a mouth or “pore.” The structure that the FT catalyst is deposited upon is very small as are the pores. The FT catalysts may deactivate by a variety of mechanisms. Reasons for Fischer-Tropsch catalyst deactivation include, without limitation: the oxidation of the active metal of the FT catalyst, e.g. oxidation of cobalt to cobalt oxide; plugging of the FT catalyst pores with heavy hydrocarbons; reaction of the active metal, such as cobalt; and blocking of active sites on the surface of the FT catalyst.
Various FT catalysts used in FT reactors have a high reactivity to oxygen or water. This makes it difficult to handle the FT catalysts in the open air, without reducing the effectiveness of the FT catalysts. Many FT catalysts are commercially available in a non-reactive or non-activated state to permit the FT catalyst to be handled safely with little to no special handling requirements needed to protect the effectiveness of the FT catalyst. However, such non-reactive FT catalysts require a chemical change in their compositions, called “activation,” before the FT catalysts can be used. Activation sometimes requires that the FT catalyst be exposed to high temperatures and to one of more gases (“activation gases”). One or more of the activation gases react with the FT catalyst to put the FT catalyst in a reactive state. As mentioned in the Nomenclature section herein, activation gases may perform oxidation or reduction or may be inert. In addition, after an FT catalyst has been in use for some period of time, the FT catalyst may become less effective and require regeneration, which includes procedures that may be similar to or may differ somewhat from activation procedures. As with activation gases, regeneration gases may perform reduction, oxidation or may be inert.
Heat transfer fluids (“HTF”), or similar heating media, have been used in liquid form to heat the FT catalyst in place within an FT reactor (“in situ”) while the FT catalyst is exposed to an activation (or regeneration) gas. If an HTF is used, operating pressures required within a shell of the FT reactor may be less than steam, even if the steam is used at the same temperature that the HTF is used. However, a volume of liquid HTF's needed to fill the FT reactor can be large.
Accordingly, there are needs in the art for novel systems and methods for FT catalyst activation and regeneration. Desirably, such systems and methods enable activation or regeneration of an FT catalyst in situ.